There have been developed voltage measuring devices for measuring a voltage across a battery assembly having a plurality of series-connected cells or battery modules. See, Japanese Laid-Open Patent Publication No. 2005-003618 (hereinafter referred to as “JP2005-003618A”), Japanese Laid-Open Patent Publication No. 2007-240299 (hereinafter referred to as “JP2007-240299A”), and U.S. Patent Application Publication No. 2008/0203976 (hereinafter referred to as “US2008/0203976A1”).
The object of JP2005-003618A is to provide a power supply voltage measuring device for measuring accurate output voltages by correcting crosstalk (mutual voltage interference) between cells, which occurs in output voltages measured across respective cells due to an additional component that is shared by the cells (paragraph [0005], Abstract).
To achieve the above object, according to JP2005-003618A, opposite ends of a battery 2, which is made up of a plurality of cells 2a through 2e, and junctions between the cells 2a through 2e are connected to respective resistors 5a through 5f, which are connected in pairs through respective capacitors 6a through 6e. The output voltages across the respective cells 2a through 2e are measured through filters that share, among the cells 2a through 2e, the resistors connected to the junctions between the cells 2a through 2e. The power supply voltage measuring device includes a voltage detecting means, having photoMOS relays 7a through 7f and a voltage measuring circuit 8 for detecting voltages across the capacitors 6a through 6e corresponding respectively to the cells 2a through 2e, and a high voltage measuring CPU 1. Based on changes per unit time in the voltages across the capacitors 6a through 6e, the high voltage measuring CPU 1 calculates output voltages across the respective cells 2a through 2e before the voltages across the capacitors 6a through 6e change (Abstract). The power supply voltage measuring device is premised on the fact that the resistors 5a through 5f have identical resistance values, and the capacitors 6a through 6e have identical capacitance values (paragraph [0019]).
According to JP2007-240299A, a task is performed to provide a voltage measuring device, which is capable of increasing noise resistance thereof without delaying the measurement time (paragraph [0022], Abstract). To accomplish this task, according to JP2007-240299A, the resistance of one Rf1 of a pair of resistors Rf1, Rf2, which are connected to the first and (N+1)th voltage detecting terminals of (N+1) voltage detecting terminals, is set to a value smaller than the resistance value of the other resistor Rf2, and the total resistance value of the pair of resistors is set to an equal value in all of a plurality of filters (claim 1, Abstract). The resistances of the resistors Rf1, Rf2 are set to the relationships, Rf1+Rf2=2Rf and Rf1<<Rf2, e.g., Rf=10 kΩ, Rf1=1 kΩ, Rf2=19 kΩ. An electrostatic capacitance of a capacitor Cf, which cooperates with the resistors Rf1, Rf2 to make up an RC filter, is set to 4.7 μF (paragraph [0036]).
US2008/0203976A1 shows an arrangement in which an RC filter is used, which is made up of a plurality of resistors and a capacitor for each of cells BT1 through BT12 (FIG. 1). The patent does not refer to the resistance values of the resistors or the electrostatic capacitance of the capacitor.